The ITU's International Morse Code recommendation ITU-R M.1677-1 specifies a "Full stop (period)" as ".−.−.−", however it falls short of specifying a decimal point. Even the examples, though they illustrate fractional numbers, don't show decimal usage.
ITU's Phonetic Alphabet and Figures regulations (AP14), however, define a "Decimal point" and require it to be spoken as "DAY-SEE-MAL".
What then is the decimal in Morse code, and where is it defined?
In the International Code of Signals (an International system of maritime communication, including radiotelegraphy), AAA (over-bar) is designated for both full stop and for decimal point. (See flashing light procedure signaling, page 20.)
The web sites of Kent Morse Keys and of the AC6V reference compendium document use of the Morse letter 'R' (dit-dah-dit) as an abbreviation for 'decimal point.' 'R' can have other meanings, of course, but I have used and heard it used to represent a decimal point in numbers - particularly when specifying a frequency - for 50 years.
The American Radio Relay League, ARRL, in their code practice texts, use "R" for the decimal point. I suspect it stands for the word, "radix" (root) because the mark used to separate whole numbers from fractions in any number system (decimal, binary, etc.) is a "radix point".
